A Class-D audio amplifier is a switching amplifier or PWM amplifier. Compared to the 50% power efficiency provided by conventional linear amplifier, the Class-D amplifier usually can provide a high power efficiency over 90%. To obtain a high-SNR Class-D amplifier, a feedback loop is often included. FIG. 1 shows a schematic view of a conventional Class-D amplifier. As shown in FIG. 1, Class-D amplifier is embodied by a PWM generator 102 and a noise shaping sigma-delta modulator 101, wherein the PWM generator 102 outputs complementary signals to a power driver 103 and through a filter 104 to drive a load. The drawback of the above embodiment is that sigma-delta modulation suffers stability problem and the modulator output signal gain is less than 1.
The conventional approach uses digital differential PWM to realize the design shown in FIG. 2. However, this approach requires a very narrow pulse width to overcome the practical technical bather. Hence, in practice, the following two problems must be overcome. The first problem is how to generate such a narrow pulse at low power consumption and low cost; and the second problem occurs when the next-stage of the differential PWM output is a power driver. This is because the pulse signal is even more narrowed when passing a power driver, as a result of dead-time of the power driver and the parasitic capacitance of the MOS transistor. For example, let TP be the minimum pulse width able to pass the power driver without diminishing, and TR be the minimum time resolution to reconstruct the input signal S. The imperative issue to solve is how to realize the DAC amplifier able to satisfy the restrictions of TP and TR simultaneously.
FIG. 2 shows a schematic view of a differential interpolation pulse width modulation (iPWM) DAC disclosed by U.S. Pat. No. 9,161,122. As shown in FIG. 2, the iPWM DAC includes an iPWM module 210, a power drive stage 220 and a filter 230, wherein iPWM module 210 is connected to a digital audio input and filter 230 is connected to a terminal load 240, for example, a speaker. iPWM module 210 generates differential pulses according to the data stream from the digital audio input, power driver stage 220 provides power to terminal load 240 and filter 230 removes unwanted harmonic signals to reconstruct an analog signal outputted to terminal load 240. iPWM module 210 further includes a PWM pulse generator 211, an interpolation resolution unit 212, a self-calibration unit 213 and a differential pulse generator 214, wherein PWM pulse generator 211 converts the digital audio input to a series of time domain pulses with width; interpolation resolution unit 212 increases the time domain resolution of the pulses; self-calibration unit 213 maintains the pulse-width accuracy of interpolation resolution unit 212; and differential pulse generator 214 converts the series of time domain pulses into voltage and time domain differential form. However, in practice, to generate a pair of pulse signals precisely symmetrical with respect to the time axis and the voltage amplitude, the manufacturing process and the external wiring load must be completely equal, which is relatively difficult to achieve in the actual manufacturing process currently available.